Pressure effects on the absorption edge, refractive index, and Raman spectra of crystalline and amorphous As₂S₃

High pressure measurements of the absorption edge, refractive index, and Raman spectra of crystalline (c-) and amorphous (a-) As₂S₃ are reported. Increasing pressure causes the band gap of both solids to rapidly red-shift. For c-As₂S₃, this implies enhancement of the interlayer-interaction; the similar behavior in a-As₂S₃ argues for a locally layer-like macromolecular structure in this glass. The refractive index, observed for a-As₂S₃, increases strongly with compression, and a simple extension of the Penn-Phillips model is discussed. For c-As₂S₃, the Raman spectra provide evidence that pressure induces a smooth transition from two-dimensional to three-dimensional behavior. An intralayer multiplet disperses at high pressure due to the increasing influence of triperiodic (crystal) symmetry. The strong blue-shift of rigid-layer modes implies quadrupling of layer-layer coupling. We have also determined the explicit phonon and implicit volume-driven contributions to the temperature dependences of the quantities studied.